The present invention relates to a radio receiver, and more specifically to a radio receiver capable of good signal reception while reducing any adverse effects due to intermodulation interference. The present invention is directed more specifically to a radio receiver having at least two intermediate filters of different frequency bands disposed between a tuner and an intermediate-frequency amplifier, the intermediate filters being automatically selectable dependent on the levels of signal reception in a channel being received and an adjacent interfering channel for applying an output signal from the tuner through the selected intermediate filter to the intermediate-frequency amplifier.
2. Description of the Prior Art
As shown in FIG. 1 of the accompanying drawings, a radio receiver such as an FM radio receiver has a tuner (front end) 11, an intermediate-frequency amplifier 12, an FM detector 13, a stereo demodulator 14, and an automatic gain control (AGC) circuit 15. The tuner 11 is composed of a high-frequency amplifier circuit 11a for amplifying an input signal from an antenna AT, a local oscillator 11b, and a mixing detector 11c. The AGC circuit 15 serves to control the gain of the high-frequency amplifier circuit 11a dependent on the level of an intermediate-frequency signal extracted from the intermediate-frequency amplifier 12.
The FM radio receiver will be subjected to jamming while receiving a signal from a desired broadcasting channel if two or more strong radio waves are applied to produce the same frequency as that of the signal of the desired channel. Such jamming is called intermodulation interference, which meets the relationship: EQU f.sub.0 =2f.sub.1 -f.sub.2
where f.sub.0 is the frequency of the signal from the desired channel, f.sub.1 and f.sub.2 the frequencies of the signals from first and second channels, respectively, causing intermodulation interference. The intermodulation interference occurs when the levels of the jamming signals are higher than the level of the signal of the desired channel.
There is known a radio receiver in-which a wide-band filter or a narrow-band filter is disposed between a tuner and an intermediate-frequency amplifier. The radio receiver with the wide-band filter only allows signal interference when there is an adjacent channel of a considerably large output capability. The radio receiver with the narrow-band filter only is subjected to distorsion due to over-modulation and poor separation. To avoid these shortcomings, there has been proposed a radio receiver with an automatic intermediate-frequency-band switching capability in-which at least two intermediate-frequency filters (IF filters) disposed between a tuner and an intermediate-frequency amplifier and having different frequency bands. While there is no signal interference from any adjacent broadcasting channel, the IF filter of a wider frequency band is used to receive a signal for reproducing sounds with less distortion. When there is jamming, the IF filter of a narrower frequency band is used to receive a jamming-free signal at the cost of the quality of reproduced sounds.
The conventional FM radio receiver operates in an automatic gain control mode at all times regardless of intermodulation interference. Therefore, it has been difficult to obtain an appropriate AGC which takes into account intermodulation interference. If an AGC setting is too low, then interference occurs, and if the radio receiver has too high an AGC setting, then it suffers from poor sensitivity. Where an FM radio receiver is carried on a moving object such as an automobile, the condition of intermodulation interference tends to vary with time to produce jamming, and it is necessary to lower the signal reception sensitivity at all times for preventing such jamming. Therefore, it has been impossible for the car-mounted FM radio receiver to have good signal reception dependent on the condition of intermodulation interference as it varies from time to time.
Prior radio receivers with automatic IF-band switching capability require a level detector circuit for detecting the level of an interference signal of each of adjacent channels, and a discriminator circuit for determining whether jamming has occurred or the extent of jamming, resulting in a high production cost.
There has been proposed a radio receiver with automatic IF-band switching capability in which detector circuits are connected respectively to IF filters disposed between a tuner and an IF amplifier, and the levels of output signals from the detector circuits are compared to compare the levels of a desired signal and jamming signals for thereby determining whether there is signal interference or not. However, the proposed radio receiver fails to determine accurately whether there is signal jamming. Furthermore, since the levels of the desired and jamming signals are picked up through the IF filters on a reception signal line, limitations are imposed on the characteristics of the IF filters. Another problem is that additional IF filters complicate the construction of a level comparator circuit at a successive stage.